The use of alkali metal containing borohydrides as a purification agent to prevent deterioration of plastics due to heat or light is known. For example, U.S. Pat. Nos. 3,372,140 and 3,413,260 discuss the use of certain borohydrides, such as quaternary ammonium borohydrides, that are soluble in the plastic. However, the use of inorganic borohydrides such as alkali metal borohydrides in this result has been hampered due to the lack of solubility and dispersibility. This problem is also pointed out in the U.S. Pat. No. 3,413,260 patent. This application obviates such long standing dispersion problem with use of a stable borohydride concentrate.
While quaternary ammonium borohydrides have greater solubility than alkali metal borohydrides, the former suffer from lack of thermal stability at the elevated temperatures used in processing plastics. A further disadvantages of such quaternary compounds is that their hydride equivalent weights are higher than the alkali metal borohydrides, therefore requiring higher amounts. Also, the quaternary compunds are more expensive.
As mentioned previously, the use of alkali metal borohydrides such as sodium and potassium borohydride as purification aids and stabilizers for polymers is known. In such application, borohydrides function by reducing oxidized impurities or functional groups that are present in the polymer as manufactured, or form as the result of photochemical and oxidative processes during end use of articles fabricated from the polymer. Elimination or reduction of those impurities and prevention of their formation during end use of the polymer improves the color, odor, stability and service life of the end use fabricated articles.
Polymers can be treated with alkali metal borohydrides during the polymerization process, or subsequent to this by post-treatment of the polymer in solution or in the bulk, molten state. For example, U.S. Pat. No. 3,086,963 describes the stabilization of polyvinylalcohol against thermally induced discoloration by treating an aqueous or organic solution of the polymer with sodium borohydride. Treatment of cellulosic film with aqueous solutions of alkali metal borohydrides to improve oxidation resistance is described in U.S. Pat. No. 3,091,554.
Alkali metal borohydride treatment during the polymerization process is described for PVC in Swedish Patent 350,975; for poly(methylmethacrylate) in Japanese Pat. No. 73,15,473; for Nylon-6 in Belgium Pat. No. 622,701; and for styrene/phenolic resins in Japanese Pat. No. 72,41,102.
The treatment of bulk, molten polymers with alkali metal borohydrides during melt processing is more difficult due to the low solubility and agglomeration tendency of the alkali metal borohydrides in many polymers and the propensity for borohydride hydrolysis by reaction with adventitious moisture at the melt processing temperature. Thus, Japanese patent application No. 10,567 describes the treatment of polyethylene with alkali metal borohydrides during melt extrusion to reduce oxidized impurities and improve the odor of the molded plastic article.
Thus, it may be seen that while there are many potential advantages to employing alkali metal borohydrides as stabilizers and purification aids during molt processing of thermoplastic resins, practical use of alkali metal borohydrides in this application have heretofore been limited by the difficulty of incorporating these compounds into the molten thermoplastic resins due to poor solubility, the tendency to agglomerate and the propensity towards hydrolysis by reaction with adventitious moisture.